1. Field of the Invention
The invention relates generally to GNSS receivers and methods of operating the GNSS receivers to reduce both time to first fix and receiver radio frequency (RF) bandwidth.
2. Background Information
When acquiring GNSS satellite signals, receivers perform searches that involve uncertainties related to satellite selection, Doppler frequency and receiver oscillator frequency. Generally, the search involves cycling through all possible satellite selections, and for each satellite successively trying possible frequencies and code phase delays in an attempt to obtain frequency and code lock with the satellite signal. At each combination of frequency and code phase delay, the receiver determines correlation power to check if the satellite signal has been acquired. the process must be performed for each satellite in the search strategy, since each satellite transmits a unique code and is associated with a different relative motive with respect to the receiver. This exhaustive search is thus time consuming.
The uncertainties related to satellite selection could be solved by use of a valid Almanac of orbiting satellites, the true time of day from a clock driven by the receiver oscillator, and an approximate location of the receiver. From the Almanac, the time and the position information, the receiver can determine the satellites in view and compute their expected Doppler frequency offsets. However most receivers, after a power down cycle, do not maintain a valid position or a sufficiently accurate time. Without the Almanac, accurate time and also a valid position, the receiver must cycle through all possible satellites in its search strategy, as discussed above. Even with the Almanac, and sufficiently accurate time and position information, the search is still subject to clock frequency uncertainties, which makes even a more limited search strategy time consuming. Further, if the receiver has moved during power down, the retained position may no longer be sufficiently valid for the computation of satellites in view and/or expected Doppler frequency.
The uncertainties related to Doppler frequency and receiver oscillator frequency require that the searches are extended by ±4 kHz for Doppler and ±12 kHz for oscillator frequency. The magnitude of the oscillator frequency uncertainty is a function of the type of oscillator or timing crystal used in the receiver. The more precise (and usually more costly) the oscillator is, the less uncertainty there is in its generated frequency. The typical oscillators used in GNSS receivers have between 5 to 10 parts per million (ppm) frequency uncertainty due to temperature and aging affects, and 10 ppm equates to additional ±16 kHz Doppler uncertainty.
Contending with either or both of the Doppler and oscillator frequency and phase uncertainties results in a longer time to first fix (TTFF), which is the time required by a GNSS receiver to achieve a position solution and typically involves acquiring and tracking the signals from at least four GNSS satellites. The oscillator frequency uncertainty further requires a wider radio frequency (RF) bandwidth for tracking, which adversely affects the tracking of weaker satellite signals due to increased susceptibility to RF interfering signals.
To reduce the TTFF, known prior GPS receivers, such as the receiver described in U.S. Pat. No. 5,663,735 to Eshenbach, may utilize data that are modulated onto special purpose radio signals, such as, for example, time announcements that are modulated onto National Institute of Standards and Technology (NIST) radio signals. The GPS receivers utilize the time announcements to determine a transition time between two data bits modulated onto a GPS satellite signal that is being acquired and, in turn, determine GPS time. Further, the GPS receivers may also use frequency information that is modulated onto the NIST radio signal to remove the uncertainty in the receiver oscillator frequency, although the phase uncertainties of the oscillator driven clock remain. One of the problems with such GPS receivers, however, is that the NIST radio signals may not be available at certain locations. Further, even if NIST radio signals are available, the content modulated onto the signals does not provide information that the GPS receiver can use directly to estimate its location.